Present mobile sonar arrays include two and three dimensional hull mounted arrays and towed linear arrays. Hull mounted arrays are generally mounted in the acoustically quiet bow area and can look effectively in all horizontal directions except toward the noisy stern. These hull mounted arrays are limited in size, particularly in the athwartship dimension by the dimensions of the ship on which they are installed. Towed linear arrays, on the other hand, are not limited by a vessel's size as long as they can be bent, folded or segmented for storage on the vessel. However, linear arrays can search effectively only in an angular segment centered on a line perpendicular to themselves. They have poor directivity in the important forward and aft directions. They have essentially no vertical directivity when steered in their otherwise optimum perpendicular directions.
Flag-like arrays of vertically separated lines have been proposed but, while they provide good vertical and horizontal directivities to beams which are perpendicular to their vertical plane, they also have poor endfire directivity. Any array whose elements are confined to a vertical plane such as the linear arrays and the flag-like arrays will exhibit a left to right ambiguity in that it will not be possible to determine if a target is to the left or right of the array without the tow changing course.
A known array having horizontally separated lines for echo location of geological layers by seismic reflections has groups of transducers equally spaced from each other along identical parallel lines. In that array the hydrophones are positioned in a uniform rectangular grid that, while satisfactory for the detection of vertically oriented seismic signals, is not satisfactory as far as horizontally oriented sonar signals are concerned. Attempts to form beams in the edgefired directions with such uniformly spaced hydrophones result in a proliferation of high level side lobes which are detrimental to the signal detection function. In addition, the hydrophones in the grid-like pattern populate a rectangular area thus causing beams pointed in different horizontal directions to have different characteristics. Identification of signals is helped if one can compare the signals arriving from a series of uniform, uniformly spaced beams.